Consolidated Statements of Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-In Capital Amount	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Balance (in shares) at Dec. 31, 2016		43,141,134
Balance at Dec. 31, 2016	$ 307,178	$ 4	$ 935,671	$ 71	$ (628,568)
Increase (Decrease) in Stockholders' Equity
Net loss	(254,237)				(254,237)
Unrealized loss from available-for-sale securities	(385)			(385)
Vesting of restricted shares (in shares)		14,052
Exercise of options (in shares)		1,385,000
Exercise of options	17,477		17,477
Share-based compensation expense related to share-based awards for employee stock purchase plan	1,164		1,164
Issuance of common stock upon purchase by employee stock purchase plan (in shares)		76,280
Issuance of common stock upon purchase by employee stock purchase plan	2,550		2,550
Equity component of 2024 Notes	138,707		138,707
Equity component of deferred financing costs for 2024 Notes	(4,257)		(4,257)
Share-based compensation expense	33,839		33,839
Balance (in shares) at Dec. 31, 2017		44,616,586
Balance at Dec. 31, 2017	242,036	$ 4	1,124,630	(314)	(882,284)
Increase (Decrease) in Stockholders' Equity
Net loss	(221,337)				(221,337)
Unrealized loss from available-for-sale securities	(441)			(441)
Vesting of restricted shares (in shares)		36,852
Vesting of restricted shares	0
Exercise of options (in shares)		472,374
Exercise of options	9,107	$ 1	9,106
Exercise of warrants (in shares)		336,059
Share-based compensation expense related to share-based awards for employee stock purchase plan	796		796
Issuance of common stock upon purchase by employee stock purchase plan (in shares)		101,822
Issuance of common stock upon purchase by employee stock purchase plan	2,566		2,566
Share-based compensation expense	27,905		27,905
Balance (in shares) at Dec. 31, 2018		45,563,693
Balance at Dec. 31, 2018	60,632	$ 5	1,165,003	(755)	(1,103,621)
Increase (Decrease) in Stockholders' Equity
Net loss	(132,993)				(132,993)
Unrealized loss from available-for-sale securities	758			758
Vesting of restricted shares (in shares)		92,031
Vesting of restricted shares	$ 0
Exercise of options (in shares)	341,337	341,337
Exercise of options	$ 2,869	$ 0	2,869
Exercise of warrants (in shares)		81,104
Exercise of warrants	1,000		1,000
Share-based compensation expense related to share-based awards for employee stock purchase plan	661		661
Issuance of common stock upon purchase by employee stock purchase plan (in shares)		111,705
Issuance of common stock upon purchase by employee stock purchase plan	1,840		1,840
Share-based compensation expense	22,954		22,954
Balance (in shares) at Dec. 31, 2019		46,189,870
Balance at Dec. 31, 2019	$ (42,279)	$ 5	$ 1,194,327	$ 3	$ (1,236,614)